The present invention is directed to a lamp that has a phosphor coating and more particularly to a fluorescent lamp having a phosphor coating on an alumina layer.
Some lamps, notably fluorescent lamps, use a coating of phosphors on the interior surface of the lamp envelope that converts ultraviolet radiation to visible light suitable for the intended purpose. Many of these phosphors are activated by rare earth ions. The coatings are typically blends that include amounts of particular phosphors and their respective rare earth activators that achieve the desired lamp brightness and color rendering index (CRI). For example, the CRI of some fluorescent lamps is desirably in excess of 82 and the 100 hour brightness is at least 3000 lumens.
Examples of the phosphors used in fluorescent lamps include one or more of a europium-activated yttrium oxide (YOE, Y2O3:Eu) red phosphor with a primary emission at 612 nm, a cerium and terbium-activated lanthanum phosphate (LAP, (La,Ce,Tb)PO4) green phosphor with a primary emission at 544 nm, a europium-activated barium magnesium aluminate (BAM, BaMgAl10O17:Eu) blue phosphor with a primary emission at 455 nm, and a europium-activated strontium borophosphate (SBP, Sr6P5BO20:Eu) blue-green phosphor with a primary emission at 480 nm. The specific rare earth activators for these phosphors include europium, terbium and cerium.
The cost of the rare earth activators is relatively high and various attempts have been made to reduce their use. In fluorescent lamps, the amount of ultraviolet radiation converted to visible light by the phosphor coating is a function of coating thickness, activator levels, and phosphor particle reflectivity. One attempt to reduce the cost of the rare earth activators in a lamp was to reduce a thickness of the coating, thereby reducing the amount of rare earth activators in the lamp. However, as the thickness of the coating was reduced more of the ultraviolet radiation passed through the coating and did not produce visible light.
To compensate for this loss of visible light, a less expensive halophosphate phosphor layer was placed under the coating (between the envelope interior surface and the coating) to convert the ultraviolet radiation that passed through the coating to visible light. However, the quality of the light emitted by the lamp was reduced due to the broad band emission spectra and low quantum efficiency of halophosphate phosphors. Specifically, the halophosphate phosphor layer reduced the lamp CRI to unacceptable levels. In addition, the halophosphate phosphor layer did not maintain consistent light output over the life of the lamp resulting in poor lamp lumen maintenance.
In a further attempt to reduce the amount of rare earth activators, the thickness of the phosphor coating was reduced and a layer of alumina was added between the phosphor coating and the lamp interior surface (instead of the halophosphate phosphor layer). The alumina layer provided some ultraviolet reflectivity so that some of the ultraviolet radiation that passed through the coating was reflected back into the phosphor coating for conversion to visible light. The alumina layer included mixed phase alumina particles that reflected the unused ultraviolet radiation back into the phosphor coating to provide higher ultraviolet conversion at lower coating weights. Nevertheless, the phosphor coating on the alumina layer still had to be relatively thick in order to achieve the desired lamp brightness.